Conventionally, this kind of transmitter-receiver employs, as its ear-piece or ear-set type acoustic transducing part, (1) means which picks up vibrations of the skull caused from talking sound by an acceleration pickup set in the auditory canal (which means will hereinafter be referred to also as a bone-conducted sound pickup microphone and the speech sending signal picked up by this means will hereinafter be referred to as a "bone-conducted sound signal"), or (2) means which guides a speech or talking sound as vibrations of air by a sound pickup tube extending to the vicinity of the mouth and picks up the sound by a microphone set on an ear (which means will hereinafter be referred to also as an air-conducted sound pickup microphone and the speech sending signal picked up by this means will hereinafter be referred to as an "air-conducted sound signal").
Such a conventional transmitter-receiver of the type which sends speech through utilization of bone conduction is advantageous in that it can be used even in a high-noise environment and permits hands-free communications. However, this transmitter-receiver is not suited to ordinary communications because of its disadvantages, i.e. the clarity of articulation of the transmitted speech is so low that the listener cannot easily identify the talker, the clarity of articulation of the transmitted speech greatly varies from person to person or according to the way of setting the acoustic transducing part on an ear, and an abnormal sound as by the friction of cords is also picked up. On the other hand, the transmitter-receiver of the type utilizing air conduction is more excellent in clarity than the above but has defects that it is inconvenient to handle when the sound pickup tube is long and the speech sending signal is readily affected by ambient noise when the tube is short.
The air-conducted sound pickup microphone picks up sounds that have propagated through the air, and hence has a feature that the tone quality of the picked-up speech signals is relatively good but is easily affected by ambient noise. The bone-conducted sound pickup microphone picks up a talker's vocal sound transmitted through the skull into the ear set, and hence has a feature that the tone quality of the picked-up speech signal is relatively low because of large attenuation of components above 1 to 2 KHz, but the speech signal is relatively free from the influence of ambient noise. As a transmitter-receiver assembly for sending excellent speech (acoustic) signals through utilization of the merits of such air-conducted sound pickup microphone and bone-conducted sound pickup microphone, there is disclosed in Japanese Utility Model Registration Application Laid-Open No. 206393/89 a device that mixes the speech signal picked up by the air-conducted sound pickup microphone and the speech signal picked up by the bone-conducted sound pickup microphone.
According to this device, the speech signals from the bone conduction type microphone and the air conduction type microphone are both applied to a low-pass filter and a high-pass filter which have a cutoff frequency of 1 to 2 KHz, then fed to variable attenuators and combined by a mixer into a speech sending signal. With this configuration, low-frequency noises in the output from the air conduction type microphone which are lower than the cutoff frequency are removed, and it is possible to remove or cancel components higher than the cutoff frequency in the noise which the bone conduction type microphone is likely to pick up, such as noise produced by friction between a cord extending from the ear set and the human body or clothing, or wind noise produced by wind blowing against the ear set. Moreover, in a high-noise environment, the SN ratio of the speech sending signal can be improved by decreasing the attenuation of the bone-conducted sound signal from the low-pass filter and increasing the attenuation of the air-conducted sound signal from the high-pass filter through manual control.
With this configuration, however, when the level of noise from the air-conducted sound pickup microphone is high, frequency components higher than the cutoff frequency need to be appreciably attenuated for the purpose of attenuating the noise, and consequently, the speech sending signal is substantially composed only of the bone-conducted sound signal components, and hence is extremely low in tone quality. Moreover, the attenuation control by the variable attenuator is manually effected by an ear set user and the user does not monitor the speech sending signal; hence, it is almost impossible to set the attenuation to the optimum value under circumstances where the amount of noise varies. Furthermore, it is cumbersome to manually control the ratio of combining the speech signal from the air-conducted sound pickup microphone and the speech signal from the bone-conducted sound pickup microphone.